Testing and measuring reception is an essential part of every installation job. Digital signal measurement in particular is a daunting area, but a quality installation can be achieved if you know what to look for and use the right equipment as Tom Carnie at Solutions Group explains.

The Digital Set Top Box has been designed in such a way that it is capable of detecting and correcting data received that is in error from that transmitted using powerful error correction circuits.

In digital satellite and digital terrestrial receivers the first correction circuit is called Viterbi and is a complex software based system. The second correction circuit is called Reed Solomon and determines errors from hardware based comparison tables. (Remember the Hamming Codes of Reed Solomon Cross Interleaved Coding used in CD and Hard Drive technology?)

The digital signal has several parameters that can be measured, and by making sure these are measured, you can achieve a quality installation. Digital Carrier Power (DCP) and Carrier to Noise Ratio (C/N) are straight-forward measurements, resulting in given levels displayed in dBuV, dBmV or dBm.

Carrier to Noise Ratio
It must be considered that C/N is not a definitive stand-alone measurement. By definition it is a ratio of carrier to noise that is being measured across the bandwidth and will not detect some distortions that will have detrimental effects to the data. Figure 1 above shows how the same C/N could be measured for both waveforms, due to the distortions being hidden within the signal bandwidth. In addition instruments that only estimate C/N and do not show BER will not be accurate enough to determine good quality.

Signal to Noise Ratio (SNR)
This is an additional measurement available for QPSK. SNR is derived from a register inside the front-end electronics of the instrument. Note: When indicating SNR the Unaohm EP314 does not show a dB unit after SNR. This is because the value displayed is not absolute, but it is very accurate in relative terms. The benefit to the installer is the SNR will allow critical adjustment of the skew of the LNB.

Bit Error Ratio (BER)
Given that nowadays it is digital data we are measuring, there is a new parameter to remember. BER determines the ratio between the total data received and the data that has been corrected. The fewer errors corrected the better quality the signal.

Interpretation of the BER is not always clear especially if you’re more used to analogue voltage measurements.There is also the added complication that the BER of Satellite (QPSK) and Terrestrial (COFDM) differ, as there are many subsystems within COFDM. For the purpose of this article I will concentrate on explaining QPSK.

The QEF is the Quasi Error Free value, 2E-4 (1 part in 2 x 10 = 20,000).

This value, set by the engineers who designed the standard for digital broadcasting, is the target performance for a set top box and is measured post Viterbi or pre Reed Solomon. It is good for Set Top Box manufacturers but not so useful for installers as we shall see.

The display of CHannel BER measurement (pre Viterbi) is the number of errors the Viterbi stage was able to detect and correct. It is the number of errors present on the Channel and it is the most important parameter for an installer to consider. Remember that until we have corrected the errors, we do not know how many there are.

If you’re using Unaohm instruments the range is typically from 1.00E-2 up to xxE-8 and can be as high as xxE-22. The display of post Viterbi BER measurement (post Viterbi and pre Reed Solomon) is the number of errors detected and corrected by the Reed Solomon stage.

This number is obviously only a fraction of the initial errors coming from the Front-End as Viterbi will have corrected some of them. When CH BER is good, i.e has fewer errors, the indication of post Viterbi BER is very low and a long period of time is necessary to show the presence of an error.

The error correction system input to output is measured in the following way:

CH BER = the number of errors corrected by the Viterbi Corrector
Post Viterbi BER = the number of errors corrected by Reed Solomon Corrector
RU = uncorrected errors are the errors detected but not solved by the RS corrector
QEF = is the Quasi Error Free position of 2.00E-4

For example, when CH BER is 1E-5 the post Viterbi goes to about 1E-13 to 1E-14 (outside its measurable range), it would be necessary to wait about 24 hours to have this value displayed.

On the other hand, when post Viterbi BER equals 2E-4 (QEF), the CH BER is 2E-2 to 2E-3. This may be good for a Set Top Box but not for an installation especially in a block of flats.
If a cloud passes in front of a dish or it is raining, the CH BER dramatically falls and the signal may fail to lock.

To ensure a quality installation, the QEF (post Viterbi) is not as good a measuring point as the Channel BER, which gives the quickest assessment of the signal, and will remain good in time under varying weather conditions.

To achieve a quality picture look for a 1E-5 CH BER, or better, at the LNB output in order to obtain 1E-3 CH BER at the outlet socket. This will provide a good distribution network. The post Viterbi will display 1E-8, as that is the lowest value capable of being indicated (EP314).

When QPSK measurement was first required in field test instruments. only the CH BER was adopted. After some time, instrument makers presented QPSK measurement with post Viterbi BER only.

There reasoning was that as the Quasi Error Free target is the value for quality, and this is what should be measured. However, this measurement, as discussed above, is not dynamic enough for an installation engineer.

In order to give the installation engineer as much information as possible, Unaohm instruments offer the post Viterbi BER, in addition to the CH BER, but as you can see in the Figure 2 opposite it has less to offer compared with CH BER. The post Viterbi BER is good for STB manufacturers because they have to demonstrate the quality of the receiver at this value.

In considering the requirements for Digital Terrestrial (COFDM), much of the above is relevant. The target for quality is measured in the same way. The DCP, C/N and BER are required to ensure that a accurate installation is achieved.

Figure 3 and Figure 4 below illustrate the relationship between some groupings of the C/N, Ch BER and Pv BER for the Unaohm DaTuM 10 handheld spectrum meter. It is important to understand that every combination of carrier number, code rate, guard and constellation will result in different curves of Ch BER and Pv BER.

Having digested the contents of this article, here is what you should be measuring to get the box to work.

Tech Tip: Be aware that the Analogue level can be as much as 33dBuV above the Digital level and can overload the input to the terrestrial set top box. Typical maximum analogue input to the Digital set top box is 80dBuV.

At the input to the Digital Satellite Set Top Box:

DCP Min 47dBuV, Max 77dBuV
C/N Min 12dB
CH BER xxE-3 (xxE-5 at the LNB)

At the input to the Digital Terrestrial Set Top Box:

DCP Min 50dBuV, Max 65dBuV (note)
C/N Min 26dB
CH BER xxE-6